Several trends within the automotive industry point to the need for waterborne coatings which have good physical properties and appearance when baked at lower than normal cure temperatures. One trend within the industry has been the effort to reduce atmospheric pollution due to volatile organic solvents emitted during the painting process. One approach to this end has been to develop waterborne coating compositions which contain less organic solvents. Another way to reduce emissions and to cut energy expenditure is to lower the temperature at which the coating is baked or cured; less fuel is required to heat an oven to a lower temperature and consequently there is less carbon dioxide emitted to the atmosphere.
Another trend is the increasing use of automobile body parts made of thermoplastic olefins which deform at normal bake temperatures. Such parts are usually painted in separate operations using coatings which are different from the coatings used to paint the metallic parts of the automobile body.
Defects in automotive topcoats which occur during application are usually repaired separately and with a minimum amount of damage to the original coating. In order to avoid overbaking the original coating, it is desirable to have repair coatings which can be cured at lower bake temperatures. Currently, it is common in automotive plants to use different coatings for metallic parts and plastic parts and to repair defects, even though all are being applied to the same automobile. Since it is difficult for coatings with different compositions to have the same appearance, it is highly desirable to have one universally applicable waterborne coating that will perform well for different applications at both low and normal cure temperatures. If different coatings must be used for the different applications, then it is desirable that the coatings have compositions as similar as possible.
Waterborne automotive coatings containing water dispersible fatty acid-containing polyesters or alkyds are known. U.S. Pat. No. 5,412,023 teaches the use of a polyester polyol containing isomerized soybean fatty acid with an acid value of 34. However, this patent specifically teaches that the polyester polyol must contain sufficient ionic groups to be stably dispersed into water. U.S. Pat. Nos. 4,851,460 and 5,342,882 teach the use of a water dispersible polyester with an acid value of 86.5 made from epoxidized linseed oil. When acrylic monomers are polymerized in the presence of such a dispersion, the result is a polyester/acrylic dispersion that is stabilized by the ionic functionality of the hydrophilic polyester. In none of these cases is there any indication that resins made from fatty acid-containing polyesters would be suitable for coatings cured at low baking temperatures.
In order to decrease damage to automotive coatings due to the effects of acid rain, some coatings manufacturers have been making modifications to topcoats that make them more hydrophobic. Such modifications can cause a decrease in adhesion when another coating is applied to them to repair defects. The coating used to repair defects must be adapted to overcome any loss in adhesion caused by the hydrophobic nature of the coating being repaired.
Unfortunately, as the baking temperature for a coating is decreased, the rate and degree of curing is also decreased. This is particularly true for coatings which are cured using aminoplast crosslinking agents. A low degree of cure can result in a weakening of the physical properties of the coating, and in particular, a reduction in the degree of cure can result in a decrease in the cohesive and adhesive strength of the coating, especially when the coating is subjected to a humid environment.
Therefore, in order to obtain the advantages of a low bake temperature, a means must be found to overcome the loss of physical properties associated with a lower degree of cure. In addition, a low temperature cure waterborne coating must still have all the characteristics required of an automotive coating, such as smoothness, metallic flake orientation, and broad application latitude.